1. Field of the Invention
The present invention relates to a scanning charged-particle beam instrument such as a scanning electron microscope and, more particularly, to a scanning charged-particle beam instrument best suited for searching a specimen surface for a desired field of view and to a method of observing images of a specimen with such a scanning charged-particle beam instrument.
2. Description of the Related Art
In scanning electron microscopy, an electron beam emitted by an electron gun is focused onto a specimen by condenser lenses and an objective lens. The electron beam is scanned in two dimensions. As the specimen is irradiated with the electron beam, secondary electrons and other electrons are produced. These produced electrons are detected with a detector. The output signal from the detector is supplied to a cathode-ray tube synchronized to the scanning of the electron beam. Thus, a scanned image of the specimen is obtained.
Where an image of a specimen is observed using such a scanning electron microscope, the specimen stage is mechanically translated in the X- and Y-directions or rotated to permit the user to observe an image of a desired area on the specimen. Movements and rotations of the observed area are not limited to mechanical ones. They may also be accomplished by the image-shifting function for controlling the range in which the electron beam is deflected and by the scan rotation function for electrically rotating the direction of the two-dimensional scan of the electron beam.
Where a desired position on a specimen is brought onto the optical axis of the optical beam in this scanning electron microscope to perform close observation of the position, the area in which the electron beam is scanned in two dimensions is widened and the observation magnification is lowered. Then, the operator observes the low-magnification image and searches for the desired specimen position. Then, the stage on which the specimen is placed is moved to bring the desired specimen position onto the optical axis of the electron beam. After this sequence of operations, the area in which the beam is scanned in two dimensions is narrowed, and the magnification is made higher. The operator performs close observation of an image of the desired position on the specimen.
The image observation making use of the process steps described above is not necessary where the observation position has been previously determined. However, with respect to a specimen whose image is to be observed first, the above-described image observation steps are essential. Where a large specimen is treated, observations may be made at plural spaced positions. At this time, whenever the stage is mechanically moved, the operator must search for a desired field of view at the minimum magnification. Especially, if a specimen position of interest does not come within the field of view after the stage is moved and the search operation is performed at the minimum magnification, then the stage must be moved again, and the operator must search for a desired field of view.